1. Field of the Invention
The present invention relates to a semiconductor laser of the distributed feedback (DFB) type having a construction appropriate for single-mode oscillation.
2. Description of the Related Art
Considerable development work is underway in general on semiconductor lasers with oscillation wavelengths of 1.5 to 1.6 micrometers due to the minimal loss of light of that wavelength band in transmission in optical fibers.
If a semiconductor laser of this conventional type, i.e., a Fabry-Perot type semiconductor laser, is used for high speed modulation, it cannot maintain the wavelength monochromatically and numerous wavelengths result. If such signal light is introduced into and transmitted through an optical fiber, the light which is output from it results in degradation of the waveform because refractive indexes (and thus the propagation speeds) for respective wavelengths are different due to the differences in dispersion in the material of the optical fiber itself. Such a signal therefore ends up with a great amount of noise at the receiving side, so that it is not practical for use.
In recent years, therefore, development has been underway on DFB type semiconductor lasers. Good results have been obtained.
A DFB type semiconductor laser has, formed on the active layer itself or close thereto, a diffraction grating known as a "corrugation" or just a "grating". Light travels back and forth and resonates in the active layer under the influence of the diffraction grating.
In such a DFB type semiconductor laser, theoretically, it is considered possible to maintain monochromatic wavelength oscillation even when modulating at a high speed of several hundred M bits/sec. In practice, however, this is very difficult. The reason for this is that the corrugation in the afore-mentioned DFB type laser is formed uniformly and, therefore, the corrugation has a uniform structure without discontinuity of the corrugation. In other words, a so-called symmetric DFB type semiconductor laser is formed in which, since the losses in the two longitudinal modes symmetrically occurring on the two sides of the side center are equal, dual-mode oscillation can take place or oscillation can transfer between two resonance modes differing by just plus or minus the same wavelength from the Bragg wavelength corresponding to the period of the corrugations, resulting in unstable oscillation. Therefore, a so-called .LAMBDA./2 shift DFB type semiconductor laser (.LAMBDA.=corrugation period) has been developed to eliminate this problem.
A conventional .LAMBDA./2 shift DFB type semiconductor laser has a structure in which, seen from the side center, the corrugation of the right side or the left side is shifted by just .LAMBDA./2. The .LAMBDA./2 shift DFB type semiconductor laser can oscillate with a single mode at the Bragg wavelength. The oscillation characteristics of the .LAMBDA./2 shift DFB type semiconductor laser are extremely superior.
There are, however, considerable problems in the manufacture of the .LAMBDA./2 shift DFB type semiconductor laser. Specifically, the period .LAMBDA. of the corrugation itself is as small as 0.3 to 0.4 micrometers, for example. Therefore, it is very difficult to manufacture the right and left two corrugations being shifted by exactly .LAMBDA./2 and being combined at the middle of the DFB laser without discontinuity of the corrugations.
To realize the same effect as in the conventional .LAMBDA./2 shift DFB type semiconductor laser, the IEEE Journal of Quantum Electronics, September 1976, page 534 suggests changing the thickness of the optical guide over a length short compared with the total length of the structure. This technique, however, also has a disadvantage in manufacturing, because it is very difficult to accurately control the thickness of the optical guide layer during crystal growing of the optical guide layer.